This invention relates to polymeric compositions that are processible and may be photochemically cured to produce crosslinked compositions useful in coating, sealants, adhesive and many other applications
For coating, sealant and adhesive applications, much research has focused on acrylic pressure sensitive adhesives (PSAs), which exhibit good adherence to high energy (i.e., polar) substrates such as metal and painted steel surfaces and which have good performance properties at high temperatures (i.e., 100xc2x0 C. or greater), are known in the art. Crosslinking acrylic compositions so as to form crosslinked PSAs is an area of much interest and research.
Solvent-processed acrylic PSA compositions can be crosslinked through the addition of polyfunctional crosslinking agents that react with functionalities within the polymer. See, e.g., Japanese Kokoku No. 58[1983]-046236. However, such processes result in the emission of solvent vapors.
The difficulty of additional processing steps, necessary to incorporate polyfunctional crosslinking agents into acrylic PSAs, can be avoided by the use of latent crosslinking reactions. This technique is exemplified in U.S. Pat. No. 4,812,541, where synergistic amounts of an N-vinyl lactam monomer and a glycidyl monomer are incorporated into an acrylate polymer to provide a high performance PSA; however, these latent crosslinked polyacrylates require post-curing that requires additional heat and/or time. Pendent functional group-containing polymers are also described in U.S. Pat. Nos. 4,908,229, 5,122,567, and 5,274,063.
The problems associated with solvent processing and crosslinking bulk-processed acrylate PSAs can be avoided through the use of actinic radiation processing. PSAs made by photopolymerizing an alkyl acrylate and a polar copolymerizable monomer (e.g., acrylic acid, N-vinyl pyrrolidone, etc.) are known in the art. See, e.g., U.S. Pat. Nos. RE 24,906, 4,181,752, 4,364,972, and 4,243,500. The cohesive strength of an acrylic PSA prepared in this manner can be increased if a photoactive crosslinking agent such as an aldehyde, a quinone, or a chromophore-substituted halomethyl-s-triazine is used in conjunction with a photoinitiator. See, e.g., U.S. Pat. Nos. 4,329,384, 4,330,590, 4,391,687, and 5,202,361. However, this type of photocrosslinking process is affected by the thickness of the composition.
In addition to actinic radiation processing, acrylate PSAs can be applied to substrates by solvent and hot-melt coating techniques. Although solvent coating techniques are widely used, hot-melt coating techniques provide some environmental and economical advantages. However, unlike solvent coating techniques where the polymer coating and crosslinking are performed simultaneously, hot-melt coating requires that coating and crosslinking be performed sequentially. This is due to competing considerations a polymer cannot be hot-melt coated effectively if it is crosslinked, yet the polymer needs to be crosslinked to achieve certain desirable performance properties (e.g., cohesive strength where the polymer is a PSA). Therefore, hot-melt coating is performed prior to crosslinking of the coated polymer.
Because hot-melt coating techniques involve high amounts of thermal energy and shear, the subsequent crosslinking procedure usually involves non-thermal energy sources. Electron beam (E-beam) and ultraviolet (UV) energy sources have been used traditionally, although E-beam techniques often are too energy intensive to be practical. Accordingly, much interest has been focused on UV radiation techniques.
U.S. Pat. No. 5,741,543 (Winslow et al.) describes a syrup polymer process in which a composition containing monomers is coated onto a substrate and crosslinked so as to form a PSA by means of polymerizing free radically polymerizable monomers from covalently attached pendent unsaturation in the polymer component of the composition. The coating can be carried out by a wide variety of industrial methods because the process of the invention allows for compositions with a wide degree of possible viscosities.
Briefly, the present invention provides novel melt-processible compositions prepared from a first polymer having a plurality of pendent polymerizable functional groups and a polymeric photoinitiator. The composition may further comprise a second component with co-reactive pendent polymerizable groups. The functional groups are reactive by free-radical addition (e.g. free radical addition to a carbon-carbon double bond). In addition, reactions involving polymeric reactants of the instant invention are controlled and precise in that they result in polymer-polymer coupling reactions only by reaction between the pendent free-radically polymerizable functional groups. The novel composition has been discovered to provide low shrinkage, low residual compositions whose properties are easily tailored to the desired end-uses. Residuals include monomers, solvents, or other volatile components.
In one aspect this invention provides crosslinkable composition comprising
a) a first component having a plurality of pendent free-radically polymerizable functional groups;
b) a polymeric photoinitiator, and
c) a residual content of less than 2.0 wt. %, preferably less than 1.0 wt. %, most preferably less than 0.1 wt. %,
wherein said composition is melt processible at temperatures of less than or equal to 100xc2x0 C.
The first component is selected from:
1) a first polymer having a plurality of pendent polymerizable functional groups, and
2) a first polyfunctional compound having a plurality of pendent polymerizable functional groups.
The composition may further comprise a second component that may be selected from a second polymer having a plurality of pendent polymerizable functional groups, and a second polyfunctional compound having a plurality of pendent polymerizable functional groups.
In another aspect this invention provides a UV crosslinkable composition that produces no or minimal by-products, and that achieves crosslink density by chain-growth addition. This invention has several advantages. The composition is low in viscosity, readily melt processible and coatable, and has minimal residuals content such as solvents, monomers, plasticizers and/or viscosity modifiers. The compositions can rapidly and reliably be prepared without requiring specialized equipment and without generating concerns about potentially toxic or irritating unreacted low molecular weight monomeric species.
The compositions may be used as coatings, including hard surface coatings, clear coatings, powder coatings and pattern coatings; as adhesives, including pressure sensitive adhesives and hot melt adhesives; as sealants; as optical coatings; as blown microfibers (BMF); as high refractive index optical materials; as barrier films; in microreplication; as low adhesion backsizes, (LABs) and as release coatings.
As used herein, the term xe2x80x9cmelt processiblexe2x80x9d or simply xe2x80x9cprocessiblexe2x80x9d is used to refer to polymer compositions that possess or achieve a suitable low viscosity for coating or extrusion at temperatures less than or equal to 100xc2x0 C., using conventional extrusion equipment without the need for addition of solvents, monomers, plasticizers and/or viscosity modifiers and without the need for extraordinary pressures.
The crosslinked compositions are useful as adhesives, including pressure sensitive adhesives, as sealants, as foams and as coatings. In one embodiment the invention provides an adhesive article comprising the crosslinked composition coated on a substrate, such as a tape backing. The novel compositions of the present invention cure by means of polymerizable functional groups to form crosslinked compositions possessing tailorable properties such as shear, peel, release and strength through selection of the particular constituents, and by control of the crosslink density. The crosslink density is predetermined by the percentage of free-radically polymerizable functional groups incorporated into the crosslinkable composition.
In another aspect this invention provides a process for making a substrate bearing a coating of a crosslinked composition (such as a pressure-sensitive adhesive) on at least one surface thereof, comprising the steps of:
a) coating the crosslinkable composition of the invention onto a substrate, and
b) subjecting said coated crosslinkable composition to sufficient energy to activate said initiator and to crosslink said composition.
For performance, environmental, and economic considerations, photoinitiated polymerization is a particularly desirable method for preparing a pressure sensitive adhesive (psa) directly on the tape backing (or release liner in the case of a so-called transfer tape in which the psa is ultimately transferred to a substrate instead of a tape backing to provide for adhesion of the bonded article or adherend). With this bulk polymerization technique, it is advantageous to create a composition having coatable viscosity of 50,000 centipoise or less (when measured at or below 100xc2x0 C.), coat the composition on the substrate, then crosslink the components to build strength and adhesive properties.
Advantageously, the present invention provides crosslinkable compositions that are readily processed without appreciable residual content (as in syrup polymer compositions). Curable systems containing monomeric species or solvent can give rise to a significant increase in density when transformed from the uncured to the cured state causing a net shrinkage in volume. As is well known, shrinkage can cause a general loss of adhesion in many instances as well as significant movement and unpredictable registration in precise bonding operations such as those required in microcircuit applications.
The composition of the present invention minimizes shrinkage due to solvent evaporation and/or monomer polymerization. The low shrinkage compositions of this invention are particularly useful in dental, molding applications or in any application where accurate molding and/or registration is required. The present invention provides a new class of reactive polymers that may be formulated as 100% solids, cured by photochemical means and that exhibit properties that meet or exceed those of solvent-borne or syrup polymers. The present invention provides compositions that exhibit less than 2% shrinkage, and preferably less than 1%.
Further, the purity of the materials and clean environment for processing are also important to produce high performance materials. Polymers used for coatings and adhesives are often desirably delivered without significant amounts of volatile materials (such as monomeric species) to eliminate any contamination. However, the problems of residual volatile materials constitute a much more formidable challenge especially when acceptable limits of migratable, volatile impurities are on the order of a few parts per million. Industries such as medical and food packaging require materials of high purity and lower cost. The composition of the present invention avoids problems due to species contamination.
The present invention provides crosslinkable compositions useful in the preparation of adhesives, coatings and sealants. The compositions are prepared from polymers having free-radically polymerizable, pendent functional groups and are formed from ethylenically unsaturated monomers. The compositions comprise a crosslinkable mixture comprising:
a) a first component having a plurality of pendent free-radically polymerizable functional groups;
b) a polymeric photoinitiator, and
c) a residual content of less than 2.0 wt. %, preferably less than 1.0 wt. %, most preferably less than 0.1 wt. %, wherein said composition is melt processible at temperatures of 100xc2x0 C. or less.
The first component is selected from:
1) a first polymer having a plurality of pendent polymerizable functional groups, and
2) a first polyfunctional compound having a plurality of pendent polymerizable functional groups.
The first polymer comprises a polymer of at least 500 Mn comprising:
(1) from 0.01 to 99.99 parts by weight of polymerized units of free radically polymerizable ethylenically-unsaturated monomers, and
(2) from 99.99 to 0.01 parts by weight of a polymerized monomer units derived from an ethylenically-unsaturated monomer possessing polymerizable functional groups.
The free radically polymerizable ethylenically-unsaturated monomers of the first polymer (having a plurality of pendent polymerizable functional groups) may comprise any of the free radically polymerizable ethylenically-unsaturated monomers, examples of which include one or more of the vinyl aromatic monomers such as styrene, xcex1-methylstyrene, 2-and 4-vinyl pyridine, and the like; xcex1,xcex2-unsaturated carboxylic acids and their derivatives such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, ethyl acrylate, butyl acrylate, iso-octyl acrylate, octadecyl acrylate, cyclohexyl acrylate, tetrahydrofurfuryl methacrylate, phenyl acrylate, phenethyl acrylate, benzyl methacrylate, xcex2-cyanoethyl acrylate, maleic anhydride, diethyl itaconate, acrylamide, methacrylonitrile, N-butylacrylamide, and the like; vinyl esters of carboxylic acids such as vinyl acetate, vinyl 2-ethylhexanoate, and the like; vinyl halides such as vinyl chloride, vinylidene chloride, and the like; N-vinyl compounds such as N-vinylpyrrolidone, N-vinylcaprolactam, N-vinylcarbazole, and the like; vinyl ketones such as methyl vinyl ketone and the like.
A preferred embodiment of the first polymer comprises
(1) from 75.00 to 99.99 parts by weight of polymerized monomer units derived from (meth)acrylic acid esters of non-tertiary alkyl alcohols containing 1-14 carbon atoms;
(2) from 0.01 to 5.00 parts by weight of polymerized monomer units derived from an ethylenically-unsaturated monomer possessing polymerizable functional groups;
(3) from 0 to 10 parts by weight of at least one polar monomer; (i.e. xe2x80x9cpolar monomersxe2x80x9d) and
(4) from 0 to 10 parts by weight of other monomers (described below).
The composition of this invention may further comprise a second component of the composition that may be a second polymer having polymerizable functional groups, or may be a second polyfunctional compound having a plurality of co-reactive functional groups. The crosslinked composition of the invention results from the polymerization of the pendent functional groups of the first and second components, with the polymeric photoinitiator.
The first polymer component of the composition comprises (and the second polymer component if present) independently comprises one or more pendent groups that include free-radically polymerizable unsaturation. Preferred pendent unsaturated groups include (meth)acryloyl, (meth)acryloxy, propargyl, and (meth)acrylamido. Such pendent groups can be incorporated into the polymer in at least two ways. The most direct method is to include among the monomer units of ethylene di(meth)acrylate, 1,6-hexanediol diacrylate (HDDA), or bisphenol-A di(meth)acrylate. Useful polyunsaturated monomers include allyl, propargyl, and crotyl (meth)acrylates, trimethylolpropane triacrylate, pentaerythritol triacrylate, and allyl 2-acrylamido-2,2-dimethylacetate.
Using the xe2x80x9cdirect methodxe2x80x9d of incorporating the pendent, free-radically polymerizable functional group, useful functional monomers include those unsaturated aliphatic, cycloaliphatic, and aromatic compounds having up to about 36 carbon atoms that include a functional group capable of free radical addition such as those groups containing a carbon-carbon double bond including vinyl, vinyloxy, (meth)acrylic, (meth)acrylamido, and acetylenic functional groups.
Examples of polyethylenically unsaturated monomers that can be used include, but are not limited to, polyacrylic-functional monomers such as ethylene glycol diacrylate, propylene glycol dimethacrylate, trimethylolpropane triacrylate, 1,6-hexamethylenedioldiacrylate, pentaerythritol di-, tri-, and tetraacrylate, and 1,12-dodecanedioldiacrylate; olefinic-acrylic-functional monomers such as allyl methacrylate, 2-allyloxycarbonylamidoethyl methacrylate, and 2-allylaminoethyl acrylate; allyl 2-acrylamido-2,2-dimethylacetate; divinylbenzene; vinyloxy group-substituted functional monomers such as 2-(ethenyloxy)ethyl (meth)acrylate, 3-(ethynyloxy)-1-propene, 4-(ethynyloxy)-1-butene, and 4-(ethenyloxy)butyl-2-acrylamido-2,2-dimethylacetate, and the like. Useful polyunsaturated monomers, and useful reactive/co-reactive compounds that may be used to prepare a polymer having pendent unsaturation are described in greater detail in U.S. Pat. No. 5,741,543 (Winslow et al.), incorporated in its entirety herein by reference.
Preferred polyunsaturated monomers are those where the unsaturated groups are of unequal reactivity. Those skilled in the art recognize that the particular moieties attached to the unsaturated groups affect the relative reactivities of those unsaturated groups. For example, where a polyunsaturated monomer having unsaturated groups of equal reactivity (e.g., HDDA) is used, premature gellation of the composition must be guarded against by, for example, the presence of oxygen, which acts as a radical scavenger. Conversely, where a polyunsaturated monomer having unsaturated groups of differing reactivities is used, the more reactive group (such as (meth)acrylate as (meth)acrylamido) preferentially is incorporated into the polymer backbone before the less reactive unsaturated group (such as vinyl, allyl, vinyloxy, or acetylenic) reacts to crosslink the composition. The direct method is generally not preferred due to difficulty in control of branching and premature gellation.
An indirect, but preferred, method of incorporating pendent groups that comprise polymerizable unsaturation into the first polymer is to include among the monomer units of the polymer some that comprise a reactive functional group. Useful reactive functional groups include, but are not limited to, hydroxyl, amino (especially secondary amino), oxazolonyl, oxazolinyl, acetoacetyl, carboxyl, isocyanato, epoxy, aziridinyl, acyl halide, and cyclic anhydride groups. Preferred among these are carboxyl, hydroxyl and aziridinyl groups. These pendent reactive functional groups are reacted with unsaturated compounds that comprise functional groups that are co-reactive with the reactive pendent functional group. When the two functional groups react, a polymer with pendent unsaturation results.
Using the xe2x80x9cindirect methodxe2x80x9d of incorporating the pendent, free-radically polymerizable functional groups, useful reactive functional groups include hydroxyl, secondary amino, oxazolinyl, oxazolonyl, acetyl, acetonyl, carboxyl, isocyanato, epoxy, aziridinyl, acyl halide, vinyloxy, and cyclic anhydride groups. Where the pendent reactive functional group is an isocyanato functional group, the co-reactive functional group preferably comprises a secondary amino or hydroxyl group. Where the pendent reactive functional group comprises a hydroxyl group, the co-reactive functional group preferably comprises a carboxyl, isocyanato, epoxy, anhydride, or oxazolinyl group. Where the pendent reactive functional group comprises a carboxyl group, the co-reactive functional group preferably comprises a hydroxyl, amino, epoxy, isocyanate, or oxazolinyl group. Most generally, the reaction is between a nucleophile and electrophic functional groups.
Representative examples of useful co-reactive compounds include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 2-(2-hydroxyethoxy)ethyl (meth)acrylate; aminoalkyl (meth)acrylates such as 3-aminopropyl (meth)acrylate and 4-aminostyrene; oxazolinyl compounds such as 2-ethenyl-1,3-oxazolin-5-one and 2-propenyl-4,4-dimethyl-1,3-oxazolin-5-one; carboxy-substituted compounds such as (meth)acrylic acid and 4-carboxybenzyl (meth)acrylate; isocyanato-substituted compounds such as isocyanatoethyl (meth)acrylate and 4-isocyanatocyclohexyl (meth)acrylate; epoxy-substituted compounds such as glycidyl (meth)acrylate; aziridinyl-substituted compounds such as N-acryloylaziridine and 1-(2-propenyl)-aziridine; and acryloyl halides such as (meth)acryloyl chloride.
Preferred functional monomers have the general formula 
wherein R1 is hydrogen, a C1 to C4 alkyl group, or a phenyl group, preferably hydrogen or a methyl group; R2 is a single bond or a divalent linking group that joins an ethylenically unsaturated group to polymerizable or reactive functional group A and preferably contains up to 34, preferably up to 18, more preferably up to 10, carbon and, optionally, oxygen and nitrogen atoms and, when R2 is not a single bond, is preferably selected from 
in which R3 is an alkylene group having 1 to 6 carbon atoms, a 5- or 6-membered cycloalkylene group having 5 to 10 carbon atoms, or an alkylene-oxyalkylene in which each alkylene includes 1 to 6 carbon atoms or is a divalent aromatic group having 6 to 16 carbon atoms; and A is a functional group, capable of free-radical addition to carbon-carbon double bonds, or a reactive functional group capable of reacting with a co-reactive functional group for the incorporation of a free-radically polymerizable functional group.
It will be understood in the context of the above description of the first and second polymers, that the ethylenically-unsaturated monomer possessing a free-radically polymerizable group is chosen such that it is free-radically polymerizable with itself (i.e. with another functional group on the same polymer) and with the pendent functional group of the second component (if present). The reactions between functional groups provide a crosslink by forming a covalent bond by free-radical addition reactions of ethylenically-unsaturated groups between polymeric compounds. In the present invention the pendent functional groups react by an addition reaction in which no by-product molecules are created, and the exemplified reaction partners react by this preferred mode.
Where the crosslinkable composition is to be processed using high temperatures and the direct method of including pendent unsaturation has been used, care must be taken not to activate those pendent groups and cause premature gelation. For example, hot-melt processing temperatures can be kept relatively low and polymerization inhibitors can be added to the mixture. Accordingly, where heat is to be used to process the composition, the above described indirect method is the preferred way of incorporating the pendent unsaturated groups.
Polymers of the present invention have a degree of polymerization generally less than about 300. The greater than expected viscosity (for polymers having a degree of polymerization greater than 300), is attributed to entanglements of polymer chains. It has been shown empirically that polymers with less than 300 repeat units are not entangled. Prior to the present invention, unentangled polymers have been shown to be processible but they have low strength. The polymers having relatively low molecular weight, then build molecular weight (and strength) by chain-growth addition of the polymers, through the pendent polymerizable functional groups.
As result of the relatively low molecular weight, the polymers are easily processible in operations such as coating, spraying, extrusion and injection molding, because of the low melt viscosity prior to crosslinking. With the present polymers, the slope of the log-log plot of viscosity vs. molecular weight (Mn) is about 1, whereas for high molecular weight polymers the slope is 3.4. The polymers of the present invention provide processibility, then crosslinking of the polymers provides the needed physical properties such as toughness, hardness, impact resistance and others that are manifested in the cured state. Unless other indicated molecular weight will refer to number average molecular weight.
The molecular weight (average) of the polymer is less than 500,000, and more preferably less than 100,000. Above this molecular weight the viscosity of the polymer is such that coating is very difficult without the use of solvents, viscosity modifiers, plasticizers or by using a xe2x80x9csyrup polymerxe2x80x9d technique, by which the polymer is dissolved in the component monomers, which react into the polymer backbone, further increasing the molecular weight. Molecular weight may be controlled through the use of chain transfer agents, such as are known in the art.
Monomers that are useful and that comprise the major portion of the first (and second if present) polymers are predominantly alkyl (meth)acrylate esters. Alkyl (meth)acrylate ester monomers useful in the invention include straight-chain, cyclic, and branched-chain isomers of alkyl esters containing C1-C14 alkyl groups. Due to Tg and sidechain crystallinity considerations, preferred alkyl (meth)acrylate esters are those having from C5-C12 alkyl groups, although use of C1-C4 and C13-C14 alkyl groups are also useful if the combinations provide a molecule averaged number of carbon atoms between C5 and C12. Useful specific examples of alkyl (meth)acrylate esters include: methyl acrylate, ethyl acrylate, n-propyl acrylate, 2-butyl acrylate, iso-amyl acrylate, n-hexyl acrylate, n-heptyl acrylate, isobornyl acrylate, n-octyl acrylate, iso-octyl acrylate, 2-ethylhexyl acrylate, iso-nonyl acrylate, decyl acrylate, undecyl acrylate, dodecyl acrylate, tridecyl acrylate, and tetradecyl acrylate. Most preferred (meth)acrylate esters include iso-octyl acrylate, 2-ethylhexyl acrylate, and isobornyl acrylate.
The first polymer (and second, if present) may comprise free-radically polymerizable monomer units derived from monomers having pendent fluorinated groups. Such xe2x80x9cfluorinated monomersxe2x80x9d are used in amounts sufficient to impart the desired degree of low surface energy and/or release properties to the resulting crosslinked composition, and are of the formula: 
wherein
R4 is hydrogen, halogen, or straight chain or branched chain alkyl containing 1 to about 4 carbon atoms;
each R5 is independently hydrogen or straight chain or branched chain alkyl containing 1 to about 4 carbon atoms;
each Q is a covalent bond or an organic linking group, such as a alkyleneoxycarbonyl group, or a sulfonamidoalkylene group;
Rf is a fully or partially fluorinated fluoroaliphatic group, such as xe2x80x94(CF2)3CF3.
A salient component of the fluorochemical monomers is the fluoroaliphatic group, designated herein as Rf. The fluorinated monomers contain from about 5 percent to about 80 percent, more preferably from about 20 percent to about 65 percent, and most preferably about 25 percent to about 55 percent fluorine by weight, based on the total weight of the compound, the loci of the fluorine being essentially in the Rf groups. Rf is a stable, inert, non-polar, preferably saturated, monovalent moiety which is both oleophobic and hydrophobic. Rf preferably contains at least about 3 carbon atoms, more preferably 3 to about 20 carbon atoms, and most preferably about 4 to about 14 carbon atoms. Rf can contain straight chain, branched chain, or cyclic fluorinated alkylene groups or combinations thereof or combinations thereof with straight chain, branched chain, or cyclic alkylene groups. Rf is preferably free of polymerizable olefinic unsaturation and can optionally contain catenary heteroatoms such as divalent oxygen, or trivalent nitrogen.
It is preferred that Rf contain about 35% to about 78% fluorine by weight, more preferably about 40% to about 78% fluorine by weight. The terminal portion of the Rf group contains a fully fluorinated terminal group. This terminal group preferably contains at least 7 fluorine atoms, e.g., CF3CF2CF2xe2x80x94, (CF3)2CFxe2x80x94, or the like. Perfluorinated aliphatic groups (i.e., those of the formula CxF2x+1, where x is 4 to 14 are the most preferred embodiments of Rf.
The fluoroaliphatic group Rf is linked to the organic portion (i.e. the oligomeric backbone or the unsaturated portion of the monomer) by a linking group designated as Q in the formulas used herein. Q is a linking group that is a covalent bond, divalent alkylene, or a group that can result from the condensation reaction of a nucleophile such as an alcohol, an amine, or a thiol with an electrophile, such as an ester, acid halide, isocyanate, sulfonyl halide, sulfonyl ester, or may result from a displacement reaction between a nucleophile and leaving group. Each Q is independently chosen, preferably contains from 1 to about 20 carbon atoms and can optionally contain catenary oxygen, nitrogen, sulfur, or silicon-containing groups or a combination thereof. Q is preferably free of functional groups that substantially interfere with free-radical oligomerization (e.g., polymerizable olefinic double bonds, thiols, easily abstracted hydrogen atoms such as cumyl hydrogens, and other such functionality known to those skilled in the art). Examples of suitable linking groups Q include straight chain, branched chain, or cyclic alkylene, arylene, aralkylene; oxy, oxo, hydroxy, thio, sulfonyl, sulfoxy, amino, imino, sulfonamido, carboxamido, carbonyloxy, urethanylene, ureylene, and combinations and multiples thereof such as sulfonamidoalkylene or polyoxyalkylene. Preferably linking group Q is a covalent bond, divalent alkylene or a sulfonamidoalkylene group.
Suitable linking groups Q include the following structures in addition to a covalent bond. For the purposes of this list, each k is independently an integer from 0 to about 20, R1xe2x80x2 is hydrogen, phenyl, or alkyl of 1 to about 4 carbon atoms, and R2xe2x80x2 is alkyl of 1 to about 20 carbon atoms. Each structure is non-directional, i.e. xe2x80x94(CH2)kC(O)Oxe2x80x94 is equivalent to xe2x80x94O(O)C(CH2)kxe2x80x94.
With reference to a formulation for a preferred embodiment of the first polymer, representative examples of free-radically polymerizable xe2x80x9cpolar monomersxe2x80x9d having at least one ethylenically unsaturated polymerizable group which are copolymerizable with acrylate and functional monomers include strongly polar copolymerizable monomers including but not limited to those selected from the group consisting of substituted (meth)acrylamides, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylonitrile, tetrahydrofurfuryl acrylate, acrylamides, and mixtures thereof, and the like.
Again, in reference to the formulation for a preferred embodiment of the first polymer, where the desired product is a psa, the selection of the xe2x80x9cother monomersxe2x80x9d useful in preparing the composition is done in such a manner that the ultimate crosslinked pressure sensitive adhesive has sufficient conformability, tack, and adhesion to form a bond to a substrate at room temperature. One measure of a psa""s ability to conform to a substrate sufficiently at room temperature and to form an adhesive bond is the material""s glass transition temperature (Tg). A useful, guiding principal is that a psa interpolymer should have a Tg of xe2x88x9215xc2x0 C. (258xc2x0 K.) or lower in order for effective adhesive application at room temperature. A useful predictor of interpolymer Tg for specific combinations of various monomers can be computed by application of Fox Equation (1) (obtained from W. R. Sorenson and T. W. Campbell""s text entitled xe2x80x9cPreparative Methods of Polymer Chemistryxe2x80x9d, Interscience: New York (1968), p. 209). Specific values for Tg""s of appropriate homopolymers can be obtained from P. Peyser""s chapter in xe2x80x9cPolymer Handbookxe2x80x9d, 3rd edition, edited by J. Brandrup and E. H. Immergut, Wiley: New York (1989), pp. VI-209 through VI-277.
Again, in reference to the formulation for a preferred embodiment of the first polymer, useful xe2x80x9cother monomersxe2x80x9d include vinyl monomers such as vinyl acetate, styrenes, and alkyl vinyl ethers; and alkyl methacrylates. Useful xe2x80x9cother monomersxe2x80x9d may also include various polyunsaturated monomers, including addition products or copolymers or polymers comprising two different functional monomers (as defined previously) such that the product/copolymer/polymer exhibits the functionality of both of the constituent starting materials/monomers.
The first component may comprise a polyfunctional compound having a plurality of pendent, free-radically polymerizable functional groups (instead of a first polymer). Useful polyfunctional compounds have an average functionality (average number of functional groups per molecule) of greater than one, preferably greater than two and most preferably greater than 3. The functional groups are chosen to be copolymerizable with the pendent functional groups on the first polymer, and are selected to be free-radically polymerizable. Useful functional groups include those described for the first polymer and include, but are not limited to vinyl, vinyloxy, acrylic and acetylenic functional groups.
Dendritic polymers are preferred polyfunctional compounds and include any of the known dendritic architectures including dendrimers, regular dendrons, dendrigrafts, and hyperbranched polymers. Dendritic polymers are polymers with densely branched structures having a large number of reactive end groups. A dendritic polymer includes several layers or generations of repeating units which all contain one or more branch points. Dendritic polymers, including dendrimers and hyperbranched polymers, can be prepared by condensation, addition or ionic reactions of monomeric units having at least two different types of reactive end groups.
Useful polyfunctional compounds have the general formula
xe2x80x83Rxe2x80x94(Z)n
where Z is a functional group such as a carbon-carbon double bond, n is greater than 1 and R is an organic radical having a valency of n. Preferably R is an alkyl radical of valency n which may be linear or branched.
The preparation and characterization of dendrimers, dendrons, dendrigrafts, and hyperbranched polymers, is well known. Examples of dendrimers and dendrons, and methods of synthesizing the same are set forth in U.S. Pat. Nos. 4,507,466; 4,558,120; 4,568,737; 4,587,329; 4,631,337; 4,694,064; 4,713,975; 4,737,550; 4,871,779 and 4,857,599, the disclosures of which are incorporated herein by reference. Examples of hyperbranched polymers and methods of preparing the same are set forth, for example, in U.S. Pat. No. 5,418,301.
More generally, dendritic polymers or macromolecules are characterized by a relatively high degree of branching (DB), which is defined as the number average fraction of branching groups per molecule, i.e., the ratio of terminal groups plus branch groups to the total number of terminal groups, branch groups and linear groups. For dendrimers, the degree of branching is one. For linear polymers the degree of branching approaches zero. Hyperbranched polymers have a degree of branching that is between that of linear polymers and ideal dendrimers. The dendritic polymers used in this invention preferably have a degree of branching which is at least equal to 0.1, more preferably greater than 0.4, and most preferably greater than 0.5.
The polymeric photoinitiator may be the same polymer or a different polymer than the first component polymer (or may be a mixture of the two) having a plurality of pendent polymerizable functional groups. In other words, the first polymer may further comprise monomer units derived from the ethylenically unsaturated monomers having a photoinitiator group. In such cases the first polymer may comprise:
(1) from 0.01 to 99.99 parts by weight of polymerized units of free radically polymerizable ethylenically-unsaturated monomers;
(2) from 99.99 to 0.01 parts by weight of a polymerized monomer units derived from an ethylenically-unsaturated monomer possessing polymerizable functional groups, and
(3) 0.1 to 5 parts by weight of ethylenically unsaturated monomers having a pendent photoinitiator group.
In a preferred embodiment, said first polymer may comprise:
(1) from 75.00 to 99.99 parts by weight of polymerized monomer units derived from (meth)acrylic acid esters of non-tertiary alkyl alcohols containing 1-14 carbon atoms;
(2) from 0.01 to 5.00 parts by weight of polymerized monomer units derived from an ethylenically-unsaturated monomer possessing polymerizable functional groups;
(3) from 0 to 10 parts by weight of polymerized monomer units derived from a polar monomer;
(4) from 0 to 10 parts by weight of polymerized monomer units derived from other monomers (as previously described) and
(5) from 0.1 to 5 parts by weight of ethylenically unsaturated monomers having a photoinitiator group.
In another preferred embodiment, the polymeric photoinitiator may be a separate polymer that comprises:
(1) from 0.01 to 99.99 parts by weight of polymerized units of free radically polymerizable ethylenically-unsaturated monomers; and
(2) from 99.99 to 0.01 parts by weight of a polymerized monomer units derived from an ethylenically-unsaturated monomer having a pendent photoinitiator group.
It will be understood with respect to the above formula, that the polymeric photoinitiator may have a photoinitiator group on essentially each repeat unit of the polymer (i.e.  greater than 90% of the repeat units).
In a preferred embodiment the separate polymeric photoinitiator comprises
(1) from 75.00 to 99.99 parts by weight of polymerized monomer units derived from (meth)acrylic acid esters of non-tertiary alkyl alcohols containing 1-14 carbon atoms;
(2) from 0 to 10 parts by weight of polymerized monomer units derived from a polar monomer; (i.e. xe2x80x9cpolar monomersxe2x80x9d);
(3) from 0 to 10 parts by weight of polymerized monomer units derived from other monomers (as previously described) and
(4) from 0.01 to 5 parts by weight of polymerized monomer units derived from ethylenically unsaturated monomers having a photoinitiator group.
Ethylenically unsaturated monomers that comprise a radiation-sensitive group, preferably an xcex1-cleaving photoinitiator group and that are copolymerizable with the aforementioned free radically-polymerizable ethylenically unsaturated monomers (hereinafter xe2x80x9cphotoinitiator monomersxe2x80x9d) constitute from 0.0001 to about 5 pbw, preferably 0.01 to 3 pbw, of the crosslinkable composition. Preferred photoinitiator monomers include free-radically polymerizable, ethylenically unsaturated compounds having the functionality represented by the structure: 
wherein R is 
wherein
Rxe2x80x2 is H or a C1 to C4 alkyl group,
R1, R2, and R3 are independently a hydroxyl group, a phenyl group, a C1 to C6 alkyl group, or a C1 to C6 alkoxy group.
A variety of photoinitiator monomers can be made by reacting an ethylenically unsaturated monomer comprising a first reactive functional group with a compound that comprises a radiation-sensitive group and second reactive functional group, the two functional groups being co-reactive with each other. Preferred co-reactive compounds are ethylenically unsaturated aliphatic, cycloaliphatic, and aromatic compounds having up to 36 carbon atoms, optionally one or more oxygen and/or nitrogen atoms, and at least one reactive functional group. When the first and second functional groups react, they form a covalent bond and link the co-reactive compounds.
Examples of useful reactive functional groups include hydroxyl, secondary amino, oxazolinyl, oxazolonyl, acetyl, acetonyl, carboxyl, isocyanato, epoxy, aziridinyl, acyl halide, and cyclic anhydride groups. Where the pendent reactive functional group is an isocyanato functional group, the co-reactive functional group preferably comprises a secondary amino, carboxyl, or hydroxyl group. Where pendent reactive functional group comprises a hydroxyl group, the co-reactive functional group preferably comprises a carboxyl, isocyanato, epoxy, anhydride, or oxazolinyl group. Where the pendent reactive functional group comprises a carboxyl group, the co-reactive functional group preferably comprises a hydroxyl, amino, epoxy, vinyloxy, or oxazolinyl group.
Representative examples of useful co-reactive compounds include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and 2-(2-hydroxyethoxy)ethyl (meth)acrylate; aminoalkyl (meth)acrylates such as 3-aminopropyl (meth)acrylate and 4-aminostyrene; oxazolinyl compounds such as 2-ethenyl-1,3-oxazolin-5-one and 2-propenyl-4,4-dimethyl-1,3-oxazolin-5-one; carboxy-substituted compounds such as (meth)acrylic acid and 4-carboxybenzyl (meth)acrylate; isocyanato-substituted compounds such as isocyanatoethyl (meth)acrylate and 4-isocyanatocyclohexyl (meth)acrylate; epoxy-substituted compounds such as glycidyl (meth)acrylate; aziridinyl-substituted compounds such as N-acryloylaziridine and 1-(2-propenyl)-aziridine; and acryloyl halides such as (meth)acryloyl chloride.
Representative examples of co-reactive compounds include functional group-substituted compounds such as 1-(4-hydroxyphenyl)-2,2-dimethoxyethanone, 1-[4-(2-hydroxyethyl)phenyl]-2,2-dimethoxyethanone, (4-isocyanatophenyl)-2,2-dimethoxy-2-phenylethanone, 1-{4-[2-(2,3-epoxypropoxy)phenyl]}-2,2-dimethyl-2-hydroxyethanone, 1-[4-(2-aminoethoxy)phenyl]-2,2-dimethoxyethanone, and 1-[4-(carbomethoxy)phenyl]-2,2-dimethoxyethanone. Such photoinitiator monomers (and polymeric photoinitiators derived therefrom) are described, for example, in U.S. Pat. Nos. 5,902,836 (Babu et al.) and 5,506,279 (Babu et al.), the disclosures of which are herein incorporated by reference.
As previously described, the composition of the present invention comprises a first component with a plurality of pendent polymerizable functional groups (which may be a first polymer or a polyfunctional compound), an optional second component with a plurality of pendent polymerizable functional groups (which may also be either a second polymer or a polyfunctional compound), and a polymeric initiator, which may be a separate polymer or may be the first polymer. The physical form of the composition may be a viscous liquid or low melting solid or a powder, which is related to the glass transition temperature and the molecular weight. The glass transition temperature and molecular weight of the polymer component(s) may be adjusted to obtain compositions having desired properties useful for a myriad of applications ranging from hot-melt adhesives to protective films. Liquid polymers may be obtained if the glass transition temperature of the polymer component (or the melting point of the polyfunctional compound) is below ambient temperature and the molecular weight of the polymer component is below entanglement molecular weight (i.e. a degree of polymerization of less than about 300). Low melting solids may be obtained when the Tg is at or below ambient temperature. Powders may be obtained when the Tg is above ambient temperature.
The first polymer can be prepared (e.g., by solution polymerization followed by isolation) and then added to a separately prepared second polymer (if present) and polymeric initiator. Any residual monomer and/or solvents used in the preparation are generally removed by conventional techniques such as distillation, vacuum evaporation, etc. The order of addition of the individual components of the composition is not important since the polymerizable functional groups do not react prior to initiation by the polymeric initiator. Thus the useful shelf life or xe2x80x9copen timexe2x80x9d is maximized, i.e. the time during which the composition is applied to a substrate (such as a tape backing) and remains sufficiently tacky to effect a bond between the first substrate and a second substrate. Once the open time has been exceeded, a second substrate cannot be readily bonded to the first substrate. Long open times are generally preferred. Shelf life refers to the amount of time the composition may be stored without premature gellation.
The composition may be coated onto backings at useful and relatively time-stable thicknesses ranging from 25-500 micrometers or more. Coating can be accomplished by any conventional means such as roller, dip, knife, or extrusion coating. Stable thicknesses are necessary to maintain the desired coating thickness prior to crosslinking of the composition to form the crosslinked composition.
A preferred method of preparing a pressure sensitive adhesive article comprises partially crosslinking the novel composition to a useful coating viscosity, coating the partially crosslinked composition onto a substrate (such as a tape backing) and further crosslinking the composition. Partial reaction provides a coatable composition in instances where the melt strength of the first polymer (and second component, if present) is too low. Useful coating viscosities are generally in the range of 500 to 10,000 cps.
Polymerization can be accomplished by exposing the composition to energy in the presence of a polymeric photoinitiator. These photoinitiators can be employed in concentrations ranging from about 0.0001 to about 5.0 pbw, preferably from about 0.001 to about 3.0 pbw, and more preferably from about 0.005 to about 0.5 pbw, per 100 pbw of the composition.
Once configured into the desired construction, the composition including the first component, optional second component and the polymeric photoinitiator may be irradiated with activating UV radiation to crosslink the composition. UV light sources can be of two types: 1) relatively low light intensity sources such as blacklights which provide generally 10 mW/cm2 or less (as measured in accordance with procedures approved by the United States National Institute of Standards and Technology as, for example, with a UVIMAP(trademark) UM 365 L-S radiometer manufactured by Electronic Instrumentation and Technology, Inc., in Sterling, Va.) over a wavelength range of 280 to 400 nanometers and 2) relatively high light intensity sources such as medium pressure mercury lamps which provide intensities generally greater than 10 mW/cm2, preferably between 15 and 450 mW/cm2. Where actinic radiation is used to fully or partially crosslink the polymer composition, high intensities and short exposure times are preferred. For example, an intensity of 600 mW/cm2 and an exposure time of about 1 second may be used successfully. Intensities can range from about 0.1 to about 150 mW/cm2, preferably from about 0.5 to about 100 mW/cm2, and more preferably from about 0.5 to about 50 mW/cm2.
Accordingly, relatively thick coatings (e.g., at least about 0.025 mm) can be achieved when the extinction coefficient of the photoinitiator is low. Coatings from of 0.5 or more mm thick are possible and are within the scope of the present invention. Additional advantages of the photopolymerization method are that 1) heating the composition is unnecessary and 2) photoinitiation is stopped completely when the activating light source is turned off.
If so desired, measuring the refractive index of the composition material especially in bulk can be used to monitor the extent of polymerization. The refractive index changes linearly with respect to conversion. This monitoring method is commonly applied in polymerization kinetics work. See discussions about the method in, for example, G. P. Gladyshev and K. M. Gibov, Polymerization at Advanced Degrees of Conversion, Keter Press, Jerusalem (1970).
When preparing a crosslinked composition of the invention, it is expedient for the photoinitiated polymerization reactions to proceed to virtual completion, i.e., depletion of the pendent polymerizable functional groups, at temperatures less than about 70xc2x0 C. (preferably at 50xc2x0 C. or less) with reaction times less than 24 hours, preferably less than 12 hours, and more preferably less than 6 hours. These temperature ranges and reaction rates obviate the need for free radical polymerization inhibitors, which are often added to acrylic systems to stabilize against undesired, premature polymerization and gelation. Furthermore, the addition of inhibitors adds extraneous material that will remain with the system and inhibit the desired polymerization of the polymer and formation of the crosslinked pressure sensitive adhesives of the invention. Free radical polymerization inhibitors are often required at processing temperatures of 70xc2x0 C. and higher for reaction periods of more than about 6 hours.
The crosslinked composition is characterized as a polymer having a first polymer chain having the residue of at least one pendent, ethylenically unsaturated moiety chemically bonded to the residue of at least one photoinitiator moiety that is pendent from a second polymer chain. Preferably each polymer chain comprises an acrylate polymer chain. Thus, during exposure to UV energy, the free radical resulting from the photoinitiator adds to the pendent ethylenically unsaturated moiety to form a crosslink between the polymer chains upon coupling or propagation with another polymerizable group on another polymer chains. When the composition further comprises a second component polymer or polyfunctional compound, the crosslinked composition is characterized as a polymer having a first polymer chain having the residue of at least one pendent, ethylenically unsaturated moiety chemically bonded to the residue of at least one photoinitiator moiety that is pendent from a second polymer chain and/or the ethylenically unsaturated moiety pendent from the second component polymer or polyfunctional compound. In general, the present crosslinked composition has effective molecular weight between crosslinks, (Mc), of greater than or equal to 1,000 and preferably greater than 3,000. Effective molecular weight between crosslinks (Mc), may be measured by dynamic mechanical analysis.
The degree of crosslinking may be easily controlled by the number and concentration of pendent unsaturated groups and by the number and concentration of photoinitiator groups that are pendent from polymer chains. The ratio of photoinitiator groups to pendent, free-radically polymerizable, unsaturated groups can vary from about 1:10,000 to 1:1, depending on the degree of crosslinking desired. Generally the smaller the Mc, the lower the elasticity and hence harder the film. On the other hand, non-crosslinked films exhibit greater flexibility.
In addition to the ingredients mentioned above, the polymer composition may include certain other materials such as pigments, tackifiers, foaming agents and reinforcing agents. However, the addition of any such material adds complexity and hence expense to an otherwise simple, straightforward, economical composition and process and is not preferred except to achieve specific results.